Exhaust-gas heat exchanger made of duplex steel

ABSTRACT

The present invention relates to an exhaust-gas heat exchanger for a motor vehicle, having an outer casing and plates arranged therein lying one above another in layers, wherein a fin plate made of austenitic steel material is arranged in an inner cassette made of ferritic steel material or made of duplex steel and at least two inner cassettes are arranged one above another like plates and the outer casing is formed from duplex steel and engages around the inner cassettes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of German Application No. 10 2014 106 807.6, filed May 14, 2014, which is incorporated herein by reference in its entirety.

The present invention relates to an exhaust-gas heat exchanger for a motor vehicle as per the features in the preamble of patent claim 1.

During the combustion of fuel, the chemical energy present in the fuel is converted on the one hand into mechanical energy and on the other hand into heat energy. In the thermodynamic cycle process performed by the ideal Carnot process, the efficiency of an internal combustion engine is limited to approximately 40%, and therefore approximately 60% of the energy present in the fuel is converted into heat and carried away with the exhaust gas or else carried away by way of the exhaust-gas flow or else the engine block of the internal combustion engine. Exhaust-gas heat exchangers are known from the prior art which are used on internal combustion engines for the most part in motor vehicles for cooling the exhaust gas which arises during the combustion of fuel or for extracting heat energy therefrom.

The exhaust gas in this case is at temperatures of more than 500° C. up to 1000° C.

For reducing pollutants in the form of exhaust-gas recirculation or else also for extracting the heat present in the exhaust gas and feeding it, for example, to a thermodynamic cycle process, it is now necessary to use an exhaust-gas heat exchanger.

An exhaust-gas heat exchanger of this type is firstly exposed directly to the high temperatures of the exhaust gas, and therefore there may be considerable thermal fluctuations during operation of the exhaust-gas heat exchanger but also in the cold-start behavior of an internal combustion engine. In addition, the exhaust gas itself has highly corrosive properties, and therefore the material and in particular the lateral surfaces and the coupling points of the exhaust-gas heat exchanger are accordingly attacked over the duration of operation.

A corresponding exhaust-gas heat exchanger is known, for example, from DE 10 2006 029 110 A1.

It is an object of the present invention to reduce the component weight and in particular the production costs of an exhaust-gas heat exchanger given an at least constant or else increased service life and also heat transfer properties.

The aforementioned object is achieved according to the invention in the case of a motor vehicle heat exchanger having the features in patent claim 1.

Advantageous embodiment variants of the present invention are the subject matter of the dependent patent claims.

The exhaust-gas heat exchanger according to the invention is for a motor vehicle and has an outer casing and plates arranged therein lying one above another in layers. Said exhaust-gas heat exchanger is characterized by the fact that an interior fin plate made of austenitized steel material is arranged in an inner cassette made of ferritic steel material or made of duplex steel and in turn at least two of the inner cassettes are arranged one above another plate-like and the outer casing is formed from duplex steel and engages around the inner cassettes.

It is thus possible to provide a plate heat exchanger which has a weight saving of up to 30%, and therefore optimum coordination is realized by the use of various materials. It is therefore possible in particular to reduce the wall thicknesses of the casing, given an at least constant strength and corresponding durability and corrosion resistance. A possible increase in price for the material used for the casing is compensated for by the lower percentage use in terms of mass, and therefore what is provided overall is an exhaust-gas heat exchanger which is highly resistant to corrosion. At the same time, the selection of materials which differ from one another makes it possible to optimally compensate for various thermal expansions, in particular also taking into consideration coupling points, in particular soldered seams, present between the components.

In the exhaust-gas heat exchanger according to the invention, a first flow duct is formed between an inner lateral surface of the casing and an outer lateral surface of the inner cassettes and a second flow duct is formed in the inner cassettes themselves. In order that the surface area for a transfer of heat is now increased within the inner cassettes, a fin plate is arranged in the inner cassette itself. The fin plate itself has an undulating or else also a sawtooth-like configuration in cross section. The fin plate is formed from austenitic steel material and is positioned in the inner cassette. In a preferred embodiment variant, the fin plate itself can also be coupled to the inner cassette, for example by an integral joining method and here in particular a soldering method. This in turn makes it possible that the heat energy taken up by way of the fin plate is passed on in particular at the coupling points to the material of the inner cassette and then from the inner cassette itself via the wall of the inner cassette to an outer lateral surface of the inner cassette and a medium flowing over it.

In order to realize simple processability and cost-effective production, the inner cassette is embodied in a two-shell form by two shells, the two shells being soldered to one another. The two shells substantially have a U-shaped configuration in cross section, these then being insertable into one another from above and below with respect to a vertical direction, so as to incorporate the fin plate, and being coupled by an integral joining method, in particular a soldering method. At the same time, it is then also possible for the fin plate to be coupled to the inner shell.

In order that two inner cassettes are then spaced apart from one another so as to form a flow duct located therebetween, the inner cassettes have outwardly directed spacing bosses. The inner cassettes which respectively lie right on the outside then bear by way of their spacing bosses against the inner lateral surface of the casing. It is optionally possible for the casing to also have inwardly directed spacing bosses. The spacing bosses here furthermore increase the surface area respectively available for the heat exchanger performance, and this further increases the cooling power. Furthermore, the ends of the inner cassettes are formed in a manner tapering toward one another. In this way, the cross section for the incoming or outgoing exhaust gas on the exhaust-gas inlet side and, respectively, the exhaust-gas outlet side is then kept as large as possible, and this proves advantageous for a very low pressure loss of the exhaust-gas heat exchanger according to the invention. By virtue of the further reduction in wall thickness on account of the use, according to the invention, of duplex steel, the pressure loss is additionally reduced, given an at least constant cooling power. The spacing bosses, which can also be in the form of bulges, expand during a soldering operation, and therefore a high-quality soldered connection is obtained.

The at least two inner cassettes are then stacked one above another and positioned in the casing. For this purpose, it is particularly preferable in turn that the casing is embodied as an outer shell in a two-shell form by two shells, the two shells being soldered to one another. For this purpose, the shells of the casing are in turn of U-shaped configuration in cross section and are insertable into one another with the respectively free ends of the U and can be coupled to one another by way of a thermal joining process, in particular a soldering process.

A duplex steel of the alloy 1.4462 or 1.4362 or 1.4162 is used with particular preference for the duplex steel of the casing and, if duplex steel is used for an inner cassette, for this too. This gives rise to an optimum combination of shaping flexibility, corrosion resistance and resistance to the action of temperature.

The soldering method used is preferably brazing, this in turn being carried out in particular in a continuous furnace or vacuum furnace. A solder filler material is used on a nickel-iron basis. This ensures a high corrosion resistance even in the case of the solder filler material. The phase equilibrium in the duplex steels is not altered by the soldering process. Ferrite and austenite are present in each case in a proportion of 50%. In particular, a soldering temperature here is between 980° C. and 1100° C.

In a further preferred embodiment variant, brackets made of duplex steel are arranged on the casing. These brackets are then used to couple the exhaust-gas heat exchanger according to the invention to an internal combustion engine. The brackets themselves can be coupled to the casing by integral joining or preferably can also be formed in a one piece and materially integral form on the casing. Within the context of the invention, in particular the components of the casing, of the inner cassette and/or of the fin plate are then produced as formed sheet metal components, in particular deep-drawn components.

Further advantages, features, properties and aspects of the present invention are the subject matter of the description hereinbelow. Preferred embodiment variants are shown in the schematic figures. These serve for the simple understanding of the invention. In the drawing:

FIG. 1 shows a longitudinal sectional view through an exhaust-gas heat exchanger, and

FIG. 2 shows a cross-sectional view through an exhaust-gas heat exchanger according to the invention.

In the figures, the same reference signs are used for the same or similar components, even if a repeated description has been omitted for reasons of simplicity.

FIGS. 1 and 2 show an exhaust-gas heat exchanger 1 in a longitudinal sectional view and, respectively, in a cross-sectional view. According to the invention, the exhaust-gas heat exchanger 1 is embodied as a plate heat exchanger, a first flow duct 2 being formed between the inner lateral surface 3 of an outer casing 4 and the outer lateral surface 5 of the inner cassettes 6 arranged in the casing 4. A second flow duct 7 is then formed in the inner cassettes 6, provision likewise being made of a respective fin plate 8 to increase the surface area available there for taking up heat. The exhaust gas A can then flow in the inner cassette 6, a corresponding quantity of heat being taken up by way of the fin plate 8 and also the inner lateral surface 9 of the inner cassette 6 and being passed on via the wall of the inner cassette 6 to a fluid (not shown in greater detail) in the first flow duct 2. According to the invention, provision is now made of a structure in such a form that the fin plate 8 is formed from austenitic material, the inner cassette 6 is formed from ferritic material or from duplex steel and the casing 4 is formed from duplex steel. The choice as to whether duplex steel or ferritic steel material is used for the inner cassette 6 is dependent on the expected vibrational loading. If relatively low vibrational loading is to be recorded, a ferritic steel material is used, whereas a duplex steel is used in the case of high expected vibrational loading.

Furthermore, the casing 4 is embodied in a two-shell form and has a first shell 10 and a second shell 11. In turn, brackets 12 are coupled to the outer side of the casing 4 for fastening the exhaust-gas heat exchanger 1 to an engine (not shown in greater detail) or an internal combustion engine. The first shell 10 and the second shell 11 are each of U-shaped configuration in cross section and are inserted into one another in an opposite direction, such that an optimum soldering gap 13 is established at a joint in order to produce a soldered seam with a particularly high quality in a later soldering method, such that this entails a high tightness and resistance to vibrations particularly under aspects of durability. The brackets 12 themselves can be formed in a one piece and materially integral form on the outer side of the casing 4 or else also can have a multi-part form, such that they are coupled to the casing 4 by means of a coupling method (not shown in greater detail). FIGS. 1 and 2 furthermore show spacing bosses 14 between the inner cassettes 6 and also between the outer inner cassette 6 and the casing 4; these spacing bosses can also be embodied in the form of a bulge or else a concavity. In particular, the spacing bosses 14 can be introduced into the respective shell of the inner cassette 6 or the casing 4 by an embossing operation. These then serve to form a spacing between the inner cassettes 6, and this in turn leads to the formation of the respective first flow duct 2. Furthermore, respectively outwardly flared ends 15, which taper toward one another, of respectively two mutually adjacent inner cassettes 6 are shown at the ends of the inner cassettes 6. These outwardly flared ends 15, too, are soldered to one another. This produces the tightness at the edge between the first flow duct 2 and the second flow duct 7. The outwardly flared ends 15 likewise expand during the soldering operation, and this leads to a good quality of the soldered connection present therebetween.

REFERENCE SIGNS

1—Exhaust-gas heat exchanger

2—First flow duct

3—Inner lateral surface of 4

4—Casing

5—Outer lateral surface of 6

6—Inner cassette

7—Second flow duct

8—Fin plate

9—Inner lateral surface of 6

10—First shell of 4

11—Second shell of 4

12—Brackets

13—Soldering gap

14—Spacing bosses

15—End of 6

A—Exhaust gas 

1. An exhaust-gas heat exchanger for a motor vehicle, having an outer casing and plates arranged therein lying one above another in layers, wherein a fin plate made of austenitic steel material is arranged in an inner cassette made of ferritic steel material or made of duplex steel and at least two inner cassettes (are arranged one above another plate-like and the outer casing is formed from duplex steel and engages around the inner cassettes.
 2. The exhaust-gas heat exchanger as claimed in claim 1, wherein a first flow duct is formed between an inner lateral surface of the casing and an outer lateral surface of the inner cassettes.
 3. The exhaust-gas heat exchanger as claimed in claim 1, wherein a second flow duct is formed in the inner cassettes.
 4. The exhaust-gas heat exchanger as claimed in claim 1, wherein the inner cassettes are embodied in a two-shell form by two shells, the two shells being soldered to one another.
 5. The exhaust-gas heat exchanger as claimed in claim 4, wherein the shells of the inner cassettes have outwardly directed spacing bosses.
 6. The exhaust-gas heat exchanger as claimed in claim 1, wherein the casing has inwardly directed spacing bosses.
 7. The exhaust-gas heat exchanger as claimed in claim 1, wherein the casing is embodied as an outer shell in a two-shell form by two shells, the two shells being soldered to one another.
 8. The exhaust-gas heat exchanger as claimed in claim 1, wherein the duplex steel used is an alloy of the number 1.4462 or 1.4362 or 1.4162.
 9. The exhaust-gas heat exchanger as claimed in claim 1, wherein brackets made of duplex steel are arranged on the casing.
 10. The exhaust-gas heat exchanger as claimed in claim 9, wherein the brackets are formed in a one piece and materially integral form on the casing. 